The present invention relates to optical communication systems and more particularly to amplification in optical communication systems.
The enormous growth in telecommunication traffic is driving the development of technology to greatly expand the available bandwidth of backbone networks. In particular, there is a great impetus towards increasing the capacity of optical communication links and reducing the costs of implementing capacity-increasing technologies.
One fruitful avenue for increasing the capacity of optical communication links has been combining optical signals with multiple wavelengths onto the same fiber, a practice referred to as wavelength division multiplexing (WDM). Key to the development of WDM systems is the development of optical amplifier technology capable of boosting optical signals in a way that is transparent to data rate and format and that can provide effective amplification across a very large bandwidth.
Continuing development of WDM systems, including the advent of so-called DWDM (dense WDM) systems is increasing the challenges inherent in optical amplifier design. Envisioned future DWDM systems will carry hundreds of very closely spaced optical signals. Spacings of less than 25 GHz are anticipated. Amplifying such a DWDM signal is problematic for a number of reasons. For example, to overcome the non-linearity distortions inherent in such close channel spacings, current Raman amplifier technology requires very high laser pump power to achieve acceptable gain in very short fibers or in fibers having a very low nonlinear coefficient, i.e., a low material nonlinear coefficient and large effective area, and consequently a very low Raman gain coefficient. The additional pump power comes at a high cost.
The very high cost of optical amplifiers capable of amplifying large numbers of closely spaced optical signals creates economic difficulties for service providers planning to install high capacity optical links. When a high capacity DWDM link is deployed, only a small portion of the available bandwidth is used initially with the rest being reserved for future expansion. A large up-front investment must therefore be made even though effective commercial exploitation of the new equipment remains quite far in the future.
What is needed are systems and methods for optical amplification that provide a very high system capacity but take into account relevant economic constraints.
An advantageous amplification architecture for DWDM systems, including systems with very high capacity, is provided by one embodiment of the present invention. A modular interleaved structure for amplification is provided. Advantages include robustness to non-linear effects, modular as-needed deployment of system capacity, and low noise figure in implementations that incorporate Raman amplification technology.
A first aspect of the present invention provides apparatus for amplification of a WDM signal. The apparatus includes a plurality of optical amplifiers, each amplifying one of a plurality of deinterleaved subband signals. Channel spacing within each of the deinterleaved subbands signals is sufficiently large to suppress non-linear crosstalk distortion by more than approximately 33 dB below a desired signal.
A second aspect of the present invention provides apparatus for amplification of a WDM signal. The apparatus includes a plurality of Raman optical amplifiers, each amplifying one of a plurality of deinterleaved subband signals. Each of the Raman optical amplifiers including fiber that is pumped with optical pump energy. A pump power level of the pump energy injected into the plurality of optical Raman amplifiers is less than or equal to approximately 350 mW per amplifier.
A third aspect of the present invention provides apparatus for amplification of a WDM signal. The apparatus includes a plurality of Raman optical amplifiers. Each of the Raman optical amplifiers amplifies one of a plurality of deinterleaved subband signals. The plurality of Raman optical amplifiers operate in a non-saturated mode.
Further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.